Airfoil for energy extracting shrouded fluid turbines

ABSTRACT

Example embodiments described herein relate an integral and/or multi-piece airfoil and a shrouded fluid turbine having the same. The airfoil can have a generally circumferential body extending circumferentially about a central axis, and can be referred to herein as a “ringed airfoil.” The airfoil can have a front portion defining a leading edge of the airfoil and a rear portion defining a trailing edge of the air foil. A cross-sectional thickness of the front portion can be non-uniform and vary along a mean camber line of the airfoil from the leading edge to a transition area. A cross-sectional thickness of the rear portion can be uniform and constant along the mean camber line of the airfoil from the transition area to the trailing edge of the airfoil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 61/383,460, filed Sep. 16, 2010, which is incorporated herein by reference in its entirety.

BACKGROUND

A fluid-driven turbine (e.g., a rotor or impeller) encircled in part or completely by one or more shrouds, ducts, or shells may be described as a shrouded fluid turbine. Examples of shrouded fluid turbines include shrouded wind turbines with wind-driven rotors and shrouded hydro turbines with water-driven rotors. Shrouded fluid turbines that are used to generate power from fluids flowing past the turbine may be described as energy extracting shrouded fluid turbines. In shrouded fluid turbines, the shroud channels the fluid past the rotor, which may increase the efficiency of the turbine.

SUMMARY

Example embodiments described herein include, but are not limited to, an airfoil for energy extracting shrouded fluid turbines with features that include a front portion having a generally circumferential body extending circumferentially about a central axis and defining a leading edge of the airfoil, and a rear portion having a generally circumferential body extending circumferentially about the central axis and defining a trailing edge of the airfoil. The front portion has a non-uniform cross-sectional thickness extending along a mean camber line of the airfoil to a transition area, and the rear portion has a substantially uniform cross-sectional thickness extending along the mean camber line from the transition area to the trailing edge.

In example embodiments, the front and rear portions can be integrally formed or can be separately formed and joined to form a multi-piece airfoil, for which the rear portion may be detachably coupled to the front portion.

In some embodiments, the front portion includes a first surface and a second surface extending between the leading edge and the transition area, and the rear portion includes a third surface and a fourth surface extending between the transition area and the trialing edge. The first and second surfaces can spaced apart from each other to form an interior area, and the third and fourth surfaces can substantially abut each other over a length of the rear portion. The interior area of the front portion can be hollow or filled with foam, and the cross-sectional thickness of the rear portion can be approximately a thickness of the third and fourth surfaces. In some embodiments, the first and third surfaces can be continuously formed and/or the second and fourth surfaces can be continuously formed.

In some embodiments, the front portion and the rear portion are formed of different materials, and/or the rear portion can comprise a non-rigid material and/or a semi-rigid material.

In some embodiments, a cross-sectional shape of the front portion can generally taper away from the leading edges and then taper towards the mean camber line to the transition area.

Example embodiments described herein include an energy extracting shrouded fluid turbine with features that include a rotor and an airfoil. The airfoil has a generally circumferential body disposed radially about the rotor, and can include a front portion and a rear portion. The front portion extends between a leading edge of the airfoil and a transition area of the airfoil, and a rear portion extending between the transition area and a trailing edge of the airfoil. The front portion has a non-uniform cross-sectional thickness extending along a mean camber line of the airfoil, and the rear portion has a substantially uniform cross-sectional thickness extending along the mean camber line.

In some embodiments, the airfoil can be an ejector shroud positioned downstream of the rotor, and the shrouded fluid turbine can include a turbine shroud that has an inlet end positioned upstream of the rotor to form a fluid flow through the turbine shroud and passed the rotor, and an exhaust end positioned downstream of the rotor and disposed within the ejector shroud. The exhaust end can include a mixing lobe to manipulate the fluid flow.

In some examples, the airfoil is a turbine shroud and the rear portion can comprise mixing lobes to manipulate fluid flow through the turbine.

In some embodiments, a cross-sectional shape of the front portion varies and can generally taper away from the leading edge and then taper towards the mean camber line to the transition area. In some embodiments, the rear portion is formed of a fabric.

Example embodiments described herein include an airfoil for an energy extracting shrouded fluid turbine with features that include a generally circumferential body extending circumferentially about a central axis and defining a leading edge and a trailing edge of the airfoil. The airfoil has a mean camber line extending between the leading edge and the trailing edge, wherein the mean camber line bisect a cross-section of the airfoil from the leading edge to the trailing edge. The airfoil has a varied cross-sectional thickness along the mean camber line between the leading edge and a transition area, wherein a varied cross-sectional shape varies and generally tapers away from the leading edge and then towards the mean camber line to the transition area. The airfoil also has a uniform cross-sectional thickness along the mean camber line between the transition area and the trailing edge, wherein the uniform cross-sectional thickness is substantially constant.

In example embodiments, a front portion of the airfoil is defined between the leading edge and the transition area, and a rear portion of the airfoil is defined between the transition area and the trailing edge. The front and rear portions can be separate and distinct components that are attached to form the airfoil and can be formed of different materials.

The summary above is provided merely to introduce a selection of concepts that are further described below in the detailed description. The summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the components, processes, and apparatuses disclosed herein may be obtained by reference to the accompanying figures. These figures are intended to illustrate the teachings taught herein and are not intended to show relative sizes and dimensions, or to limit the scope of examples or embodiments. In the drawings, the same numbers are used throughout the drawings to reference like features and components of like function.

FIG. 1 is a right front perspective view of an example embodiment of a shrouded fluid turbine.

FIG. 2 is a left rear perspective view of the shrouded fluid turbine of FIG. 1.

FIG. 3 is a side cross-sectional view of the shrouded fluid turbine of FIG. 1.

FIG. 4 is a rear view of the shrouded fluid turbine of FIG. 1.

FIG. 5 is a cross-section of an example embodiment of an airfoil suitable for use in a shrouded fluid turbine.

FIG. 6 is a cross-section of an example embodiment of a multi-piece airfoil suitable for use in a shrouded fluid turbine.

FIG. 7 is a cross-section of another example embodiment of a multi-piece airfoil suitable for use in a shrouded fluid turbine.

FIG. 8 is a cross-section of yet another example embodiment of a multi-piece airfoil suitable for use in a shrouded fluid turbine.

FIG. 9 is a cross-section of still another example embodiment of a multi-piece airfoil suitable for use in a shrouded fluid turbine.

FIG. 10 is a right front perspective view of another example embodiment of a shrouded fluid turbine.

FIG. 11 is a left rear perspective view of the shrouded fluid turbine of FIG. 10.

FIG. 12 is a side cross-sectional view of the shrouded fluid turbine of FIG. 10.

FIG. 13 is a right front perspective view of yet another example embodiment of a shrouded fluid turbine.

FIG. 14 is a left rear perspective view of the shrouded fluid turbine of FIG. 13.

FIG. 15 is a side cross-sectional view of the shrouded fluid turbine of FIG. 13.

FIG. 16 is a rear view of the shrouded fluid turbine of FIG. 13.

FIG. 17 is a right front perspective view of another example embodiment of a shrouded fluid turbine.

FIG. 18 is a left rear perspective view of the shrouded fluid turbine of FIG. 17.

FIG. 19 is a side cross-sectional view of the shrouded fluid turbine of FIG. 17.

DETAILED DESCRIPTION

Example embodiments described herein relate an integral and/or multi-piece airfoil for use in a shrouded fluid turbine. In example embodiments, an airfoil can have a generally circumferential body extending circumferentially about a central axis, and can be referred to herein as a “ringed airfoil.” The airfoil can have a front portion defining a leading edge of the airfoil and a rear portion defining a trailing edge of the air foil. A cross-sectional thickness of the front portion can be non-uniform and can vary along a mean camber line of the airfoil from the leading edge to a transition area. A cross-sectional thickness of the rear portion can be uniform, constant, and/or near constant (e.g., to account for manufacturing tolerances, material expansion and contraction and wear) along the mean camber line of the airfoil from the transition area to the trailing edge of the airfoil. The transition area can define an area in which the airfoil transitions from a non-uniform thickness to a uniform thickness. Example embodiments of the airfoil can reduce a quantity of materials necessary to produce the airfoil, while advantageously providing similar aerodynamic properties of a convention airfoil, such as a National Advisory Committee for Aeronautics (NACA) airfoil.

FIGS. 1 through 3 depict an example shrouded fluid turbine 100 that includes a turbine shroud 104, an ejector shroud 106 downstream of the turbine shroud 104, a nacelle body 116, and a rotor 110 that encircles the nacelle body 116. As illustrated by FIGS. 1 and 2, the rotor 110, turbine shroud 104, and ejector shroud 106 may be coaxial with each other, (i.e., all share the common central axis 105).

The turbine shroud 104, which also may be identified herein as a mixer shroud, a mixing shroud or a first shroud, includes a leading edge 122, also known as an inlet end or a front end, and a trailing edge 124, also known as an exhaust end or rear end. Similarly, the ejector shroud 106, which also may be identified herein as a second shroud, includes a leading edge 150 (FIG. 3), also known as an inlet end or front end, and a trailing edge 152 (FIG. 3), also known as an exhaust end or a rear end. As depicted, support members 142 may connect the turbine shroud 104 to the ejector shroud 106.

Throughout this disclosure, the front end (inlet or leading edge) of a turbine shroud may be considered the front of a shrouded fluid turbine, and the rear end (exhaust or trailing edge) of an ejector shroud may be considered the rear of the shrouded fluid turbine. A first component located closer to the front of the shrouded fluid turbine may be considered “upstream” of a second component located closer to the rear of the shrouded fluid turbine (e.g., the turbine shroud upstream of the ejector shroud). The second component may be described as “downstream” of the first component (e.g., the ejector shroud downstream of the turbine shroud). Furthermore, the terms “inner surface” is used herein to define a surface of a shroud that is inwardly facing towards a center axis of the shroud and the term “outer surface” is used herein to define a surface that is outwardly facing away from the center axis of the shroud such that the inner surface is closer to the center axis than the outer surface.

As illustrated by FIGS. 1 and 2, the rotor 110 includes one or more blades 114 connected to a central hub 117. As used herein, any description of, or reference, to “a blade” or “the blade” refers to some or all blades of a rotor. Although rotor 110 is depicted with four blades 114, in other embodiments, the rotor 110 may include two blades, three blades, four blades or more blades.

The term “rotor” is used herein to refer to any component or assembly in which one or more blades are attached to, or coupled with, a shaft and able to rotate, allowing for the extraction of energy or power from a fluid stream flow that rotates the blade(s). Example rotors include, but are not limited to, a propeller-like rotor, an impellor and a rotor/stator assembly. As understood by one skilled in the art, any type of rotor may be used in conjunction with the turbine shroud in the shrouded fluid turbine of the present disclosure.

Although turbine shroud 104 is shown encircling the rotor 110, in some example embodiments, the turbine shroud may only partially encircle the rotor (e.g., the turbine shroud may have gaps, or the rotor may extend beyond the leading edge or trailing edge of the turbine shroud). In some embodiments, the turbine shroud may not encircle the rotor (e.g., the rotor may be positioned in front of the leading edge or past the trailing edge of the turbine shroud).

As illustrated by FIG. 3, the turbine shroud 104 may have the cross-sectional shape of an airfoil with the suction side (i.e., low pressure side) on the interior of the shroud. The trailing edge 124 of the turbine shroud 104 has mixing lobes 126, 128 that may extend downstream beyond the rotor blades 114. As shown, the mixing lobes may include low energy mixing lobes 128 that extend inward toward the central axis 105, and high energy mixing lobes 126 that extend outward away from the central axis 105. In other words, the trailing edge 124 of the turbine shroud 104 is shaped to form two different sets of mixing lobes. When viewed from the rear the mixing lobes 126, 128 form a general circular crenellated or circumferential undulating in-and-out shape, as shown in FIG. 4. The turbine shroud 104 may define an opening 130 or channel located between a low energy mixing lobe 128 and a high energy mixing lobe 126 that increases mixing between high and low energy streams. A trailing edge 122 of the turbine shroud 104 may extend downstream beyond a leading edge 150 of the ejector shroud 106, as shown.

Still referring to FIG. 3, the leading edge 122 of the airfoil defined by the turbine shroud can be generally rounded, bull-nosed, or otherwise shaped to form an aerodynamic surface for dividing a fluid into at least two flows or streams. The cross-sectional shape of the airfoil can taper away from the leading edge 122 increasing in cross-sectional thickness and then can taper towards a center or mean camber line 133 decreasing in cross-sectional thickness to the trailing edge 124 along the mean camber line 133 such that the cross-sectional thickness of the airfoil formed by the turbine shroud 104 generally varies from the leading edge 122 (e.g., inlet end of the turbine shroud) to the trailing edge 124 (e.g., exhaust end of the turbine shroud). The center or mean camber line 133 is generally positioned midway between outer and inner surfaces 134 and 136, respectively, along the longitudinal extent of the airfoil.

As shown in FIG. 3, a chord 132 defines a length of the airfoil between the leading edge 122 and the trailing edge 124 of the turbine shroud 104, which can be determined based on the mean camber line 133. A cross-sectional thickness 138 of the airfoil formed turbine shroud 104 can correspond to distance from the outer surface 134 to the inner surface 136 of the turbine shroud 104, measured perpendicular to the mean camber line 133, and can vary with distance along the mean camber line 133 such that the cross-sectional thickness 138 varies over a length of the airfoil from the leading edge 122 to the trailing edge 124. An interior area 140 formed by the surfaces 134, 136 can be hollow, or can be filled with support members for providing structural rigidity and shape. In accordance with one embodiment of the present invention, a foam material 142 may be utilized in providing both shape and structural rigidity to the assembly.

Referring to FIG. 3, the ejector shroud 106 forms an airfoil having a front portion 146 and an elongate rear portion 148. The airfoil defined by the ejector shroud 106 can have a generally cylindrical or ring-like configuration having a circumferential body extending about the central axis 105. The foil can have a leading edge 150 and a trailing edge 152. The leading edge 150 may also be referred to herein as a front, intake, and/or inlet edge, such that the leading edge 150 forms an inlet end of the ejector shroud 106 through which fluid flows. The trailing edge 152 may also be referred to herein as a rear edge, outlet edge, and/or an exhaust edge such that the trailing edge 152 defines an exhaust end of the ejector shroud 104. The leading edge 150 of the ejector shroud 106 can be positioned downstream of the rotor blades 114 and upstream of the exhaust end of the turbine shroud 104 and the trailing edge 152 can be positioned downstream beyond the exhaust end of the turbine shroud 104. Alternatively, the leading edge 150 of the ejector shroud 106 can be positioned downstream of the rotor blades 114 and downstream of the exhaust end of the turbine shroud 104 and the trailing edge 152 can be positioned downstream further beyond the exhaust end of the turbine shroud 104.

The front portion 146 of the airfoil defined by the ejector shroud 106 can extend from the leading edge 150 to a transition area 154. The transition area can be a region, zone, line, location, and the like, where the airfoil transitions from a non-uniform thickness to a uniform thickness. The leading edge 150 of the front portion 142 can be generally rounded, bull-nosed, or otherwise shaped to form an aerodynamic surface for dividing a fluid flow into at least two separate flows or streams. The front portion 146 can have an outer surface 156 and an inner surface 158 extending between the leading edge 146 and the transition area 154 to form an interior area 160 having a non-uniform cross-sectional thickness 162 along a center or mean camber line (not shown) associated with the airfoil. The mean camber line generally bisects the cross-section of the airfoil midway between the outer surface of the airfoil (e.g., the surface formed by surfaces 156 and 170) and the inner surface of the airfoil (e.g., the surface formed by surfaces 158 and 172). The cross-sectional thickness 162 of the front portion 146 is measured between the surfaces 156 and 158 perpendicular to and along the mean camber line. A chord 168 defines a length of the airfoil defined by the ejector shroud 106 between the leading edge 150 and the trailing edge 152 of the airfoil, the location of which can be determined based on a mean camber line associated with the airfoil.

The cross-sectional shape of the front portion 146 generally tapers away from the leading edge 146 (increases in cross-sectional thickness) and then tapers towards the mean camber line (decreases in cross-sectional thickness) to the transition area 154. The cross-sectional thickness 162 of the front portion 146 generally varies along a length 155 of the front portion 146 from the leading edge 150 to the transition area 154. The rear portion 148 of the airfoil defined by the ejector shroud 106 can extend from the transition area 154 to the trailing edge 152. The rear portion 148 can have a substantially uniform and constant cross-sectional thickness 164 along its length 166.

In an example embodiment, outer and inner surfaces 170 and 172, respectively, of the rear portion 148 are generally disposed adjacently to each other such that there is substantially no separation between the material or materials that form the outer surface 170 and the inner surface 172 and the thickness of rear portion 148 can be approximately the thickness of the material that forms the surfaces 172 and 178.

In some embodiments, the front portion 146 and rear portion 148 of the airfoil can be integrally formed, the surface 156 can be continuously formed with the surface 170, and the surface 158 can be continuously formed with the surface 172. In some embodiments, the front portion 146 and rear portion 148 can be formed as separate component that can be joined such that the surface 156 and the surface 170 do not form a continuous surface and the surface 158 and the surface 172 do not form a continuous surface.

In some embodiments, the rear portion 148 can extend linearly from the transition area 154 to the trailing edge 152. In some embodiments, the rear portion 148 can have a curvature between the transition area 154 and the trailing edge 152 such that the diameter or width of the rear portion 148 at the trailing edge 152 is greater than the diameter or width of the rear portion 148 at the transition area 154, or vice versa.

Functionally, the shrouded fluid turbine may be described as a mixer-ejector turbine because the low energy mixing lobes 128 and the high energy mixing lobes 126 of the turbine shroud 104 together with the ejector shroud 106 form a mixer-ejector pump. A shrouded fluid turbine incorporating a mixer-ejector pump may more efficiently extract power from a fluid flow than a shrouded fluid turbine that does not include a mixer-ejector pump. A primary fluid stream is ingested (received) by the turbine shroud 104 and channeled to the rotor 110, which extracts power from the primary fluid stream. The ejector shroud 106 of the mixer-ejector pump ingests flow from the primary fluid stream as well as a higher velocity secondary fluid stream 4 that bypasses the turbine shroud 104. The mixing lobes 126, 128 of the turbine shroud 104 in combination with ejector shroud 106 promote turbulent mixing of the lower velocity primary fluid stream from the ejector shroud 104 and the higher velocity secondary fluid stream. This turbulent mixing increases the overall velocity of the combined fluid stream output from the ejector shroud 106. This turbulent mixing enhances the power output of the system by increasing the amount of fluid flow through the system, by increasing the velocity of the primary fluid stream at a plane of the rotor for more power availability, and by reducing the pressure on a down-wind side of the rotor plane.

Although turbine shroud 104 is shown encompassing or encircling the rotor 110, in some embodiments, the turbine shroud 104 can partially encompass or encircle the rotor 110 (e.g., the turbine shroud may have gaps, have slots, be discontinuous, segmented, and the like, or the rotor 110 may extend beyond the leading edge 122 or trailing edge 124 of the turbine shroud 104). Likewise, the ejector shroud may have gaps, have slots, be discontinuous, segmented, and the like. In some embodiments, the turbine shroud 104 may not encircle the rotor 110 (e.g., the rotor 110 may be positioned in front of the leading edge 122 or past the trailing edge 124 of the turbine shroud 104).

In example embodiments, the components of turbine shroud can be integrally formed or can be formed as separate components that are joined together. Likewise, the components of the ejector shroud can be integrally formed or can be formed as separate components that are joined together. As one example, the turbine shroud and/or the ejector shroud can be formed using a blow molding process in which a molten plastic parison is shaped to conform to a mold using forced air. Some advantages of this process include continuous extrusion, and multi-layer coextrusion with up to seven layers in the finished part. Cycle times can also be shorter than rotational molding.

Some examples of plastic materials that can be used to form the turbine and/or ejector shrouds can include, but are not limited to polymers, such as a polyolefin or a polyamide. Some examples of polyolefins include polypropylene and polyethylene, such as high density polyethylene (HDPE) and low density polyethylene (LDPE). Some examples of polyamides include nylons. In some embodiments, polyvinyl chloride and plastisols can be used to form the turbine and/or ejector shrouds.

As another example, the portions of the turbine and/or ejector shroud can be formed using materials such as fabrics, polymeric films, thin metal sheets, thin composites, and the like. For embodiments in which fabric used, the fabric can be impregnated with a polymer resin (such as polyvinyl chloride) or a polymer film (such as modified polytetrafluoroethylene). Some examples of polymeric films include, but are not limited to polyvinyl chloride (PVC), polyurethane, polyfluoropolymers, multi-layer films of similar composition, and the like. Polyurethane films can be durable and can have good weatherability. Aliphatic versions of polyurethane films can be generally resistant to ultraviolet radiation. Some examples of polyfluoropolymers include polyvinyldidene fluoride (PVDF) and polyvinyl fluoride (PVF). Commercial versions are available under the trade names KYNAR® and TEDLAR®. Polyfluoropolymers generally have very low surface energy, which allow their surface to remain somewhat free of dirt and debris, and can shed ice more readily as compared to materials having a higher surface energy.

In example embodiments, the material or materials used to form the turbine and/or ejector shrouds can be reinforced with a reinforcing material, such as, for example, highly crystalline polyethylene fibers, paramid fibers, and polyaramides.

The shroud material can be formed of multiple layers, for example, comprising two, three, or more layers. Multi-layer constructions may add strength, water resistance, ultraviolet (UV) stability, and other functionality.

In some embodiments, the front portion of the ejector shroud can be a sandwich composite material, such as an epoxy-impregnated e-glass matte, and the space inside the sandwich composite can be filled with foam. This configuration provides for high beam stiffness construction with overall low density. The rear portion of the ejector shroud can be a fabric that is impregnated with a polymer resin or a polymer film to provide a polymer-impregnated fabric that is light and relatively rigid.

FIG. 5 is a cross-sectional view of an example embodiment of a ringed airfoil 200 having a generally circumferential body 202 suitable for use in accordance with example embodiments of shrouded fluid turbines described herein. In an example embodiment, the airfoil 200 includes an integrally formed front and rear portions 204 and 206, respectively, and can be suitable for use with shrouded fluid turbines, such as one or more of the shrouded fluid turbines described herein. As described above, the front portion 204 of the airfoil 200 extends between the leading edge 208 and a transition area 210 and the rear portion 206 extends from the transition area 210 to the trailing edge 212. In the example embodiment, surfaces 214 and 216 extend from the leading edge 208 of the airfoil 200 to the trailing edge 212 of the airfoil 200. For example, the inner or suction side surface 216 extends from the leading edge 208 to the trailing edge 212 on the suction side (low pressure) of the airfoil 200 (i.e., inward of the perimeter of the airfoil 200 or the surface of the airfoil 200 that is closer to the central axis of the airfoil) and the outer or high pressure side surface 214 extends from the leading edge 208 to the trailing edge 212 on the high pressure side of the airfoil 200 (i.e., outward of the perimeter of the airfoil 200 or the surface of the airfoil 200 that is further from the central axis of the airfoil 200). The material or materials that form the surface 214 can have a thickness 218 and material or materials that form the surface 216 can have a thickness 220.

A chord 222 defines a length of the airfoil 200 between the leading edge 208 and the trailing edge 212 of the airfoil 200, which can be determined based on a center or mean camber line 205 associated with the airfoil 200. The cross-sectional shape of the front portion 204 generally tapers away from the leading edge 208 and then tapers along the mean camber line 205 to the transition area 210. The mean camber 205 generally bisects the cross-section of the airfoil midway between the outer surface 214 of the airfoil 200 and the inner surface 216 of the airfoil 200. A cross-sectional thickness 224 of the front portion 204 generally varies along a length of the front portion 204 from the leading edge 208 to the transition area 210. The rear portion 206 of the airfoil 200 extends from the transition area 210 to the trailing edge 212. The rear portion 206 can have a substantially uniform and constant cross-sectional thickness 224 along its length.

The cross-sectional thickness 224 of the airfoil 200 can correspond to distance from the outer surface 214 to the inner surface 216 of the airfoil 200, measured perpendicular to the mean camber line 205. In the example embodiment, the cross-sectional thickness 224 of the airfoil 200 is non-uniform and varies in the front portion along the mean camber line 205 of the airfoil 200 from the leading edge 208 to the transition area 210. In contrast, the cross-sectional thickness 224 of the airfoil 200 is substantially uniform and constant in the rear portion 206 along the mean camber line 205 of the airfoil 200 from the transition area 210 to the trailing edge 212.

The cross-sectional thickness 224 of the front portion 204 varies from the leading edge 208 to the transition area 210 such that an interior area 226 between the surfaces 214 and 216 changes from the leading edge 208 to the transition area 210. The interior area 226 between the surfaces 214, 216 in the front portion can be empty to form a hollow cavity, can include support structures, and/or filled with a material, such as foam. In some embodiments, the rear portion 206 can have a slight taper towards the mean camber line 205 at the trailing edge 212 to minimize turbulence of the fluid flows mixing beyond the trailing edge 212. In the example embodiment, the surfaces 214, 216 can be positioned adjacent to each other and can be in contact to form the rear portion 206 such that the cross-sectional thickness 224 of the rear portion 206 can be approximately equal the thickness of the material or materials that form the surfaces 214, 216.

As shown in FIG. 5, the rear portion 206 has a first cross-sectional thickness 228 that can be measured near the transition area 210 and a second cross-sectional thickness 230 that can be measured toward trailing edge 212 of the rear portion 206. In example embodiments, the cross-sectional thicknesses 228, 230 can be approximately equal to each other.

In some embodiments, the rear portion 206 can be formed from one of the surfaces 214 and 216. For example, in some embodiments, the rear portion 206 can be formed of the surface 216, which can be a sheet of material with a thickness corresponding to the thickness of the material of materials that form the surface 216, such that the thickness of the rear portion 206 can be equal to the thickness of the material that forms the surface 216. Therefore, no space or void exists in the rear portion 206.

FIG. 6 is a cross-sectional view of an embodiment of a multi-piece, ringed airfoil 300 having a circumferential body 302 suitable for use with shrouded fluid turbines, such as one or more shrouded fluid turbines described herein. In an example embodiment, a front portion 304 and a rear portion 306 of the airfoil 300 can be formed as separate components that are joined by one or more attachment members, such as bolts, screws, rivets, clamps, clips, bracket, Velcro®, hinge, adhesives, welding, and the like. In one embodiment the joint between the front portion 304 and the rear portion 306 of the airfoil can be a flexible, hinge like, joint such that active or passive relative motion between the front portion 304 and the rear portion 306 is permitted. In some embodiments, the front portion 304 and the rear portion 306 can be detachably coupled to permit removal, repair, and/or replacement of the front and/or rear portions 304 and 306, respectively, and/or to remove the rear portion 306 in response to high wind loads that may damage the turbine.

As described above, the front portion 304 of the airfoil 300 extends between the leading edge 308 and an interface surface 309 and the rear portion 306 extends from an interface surface 311 to the trailing edge 312. In some embodiments, the interface surface 311 of the rear portion 306 can interface with the interface surface 309 in the transition area 310 when the airfoil 300 is assembled such that the transition area 310 coincides with the interface surfaces 309 and 311. In some embodiments, the interface surfaces 309 and 311 can interface outside of the transition area 310 when the airfoil is assembled such that the transition area 310 does not coincide with the interface surfaces 309 and 311. In an example embodiment, surfaces 314 and 316 extend from the leading edge 308 of the airfoil 300 to the interface surface 309 of the airfoil 300 and surfaces 315 and 317 extend from the interface surface 311 to the trailing edge 312.

A chord 322 defines a length of the airfoil 300 between the leading edge 308 and the trailing edge 312 of the airfoil 300, which can be determined based on a center or mean camber line 305 associated with the airfoil 300. The mean camber line 305 generally bisects the cross-section of the airfoil 300 midway between the outer surface of the airfoil 300 (e.g., the surfaces 314, 315) and the inner surface of the airfoil 300 (e.g., the surfaces 316, 317). The cross-sectional shape of the front portion 304 generally tapers away from the leading edge 308 and then tapers towards the mean camber line 305 to the transition area 310.

A cross-sectional thickness 324 of the front portion 304 of the airfoil 300 can correspond to distance from the outer surface 314 to the inner surface 316 of the airfoil 300, measured perpendicular to the mean camber line 305. A cross-sectional thickness 324 is non-uniform and varies in the front portion along the mean camber line 305 of the airfoil 300 from the leading edge 308 to the interface surface 309. A cross-sectional thickness 325 of the rear portion 304 of the airfoil 300 can correspond to distance from the outer surface 315 to the inner surface 317 of the airfoil 300, measured perpendicular to the mean camber line 305. The cross-sectional thickness 325 of the airfoil 300 is substantially uniform and constant in the rear portion 306 along the mean camber line 305 of the airfoil 200 from the interface surface 311 to the trailing edge 312.

The cross-sectional thickness 324 of the front portion 304 varies from the leading edge 308 to the interface surface 309 such that an interior area 326 between the surfaces 314 and 316 changes from the leading edge 308 to the interface surface 309. The interior area 326 between the surfaces 314, 316 in the front portion can be empty to form a hollow cavity or filled with a material, such as foam. In some embodiments, the rear portion 306 can have a slight taper at the trailing edge 312 to minimize turbulence of the fluid flows mixing beyond the trailing edge 312.

The front portion 304 and/or the rear portion 306 can include at least one of the attachment members 301 to secure the rear portion 306 to the front portion 304 at their interface surfaces 309 and 311, respectively, to form the multi-piece airfoil 300 extending from the leading edge 308 to the trailing edge 312.

FIG. 7 illustrates another embodiment of a multi-piece ringed shaped airfoil 400. The airfoil 400 can have a generally circumferential body 402 with a front portion 404 and a rear portion 406 formed as separate components that are joined by one or more attachment members, such as bolts, screws, rivets, clamps, clips, bracket, Velcro®, hinge, adhesives, welding, and the like. The airfoil 400 can be suitable for use with fluid turbines, such as one or more fluid shrouded turbines described herein. In some embodiments, the front portion 404 and the rear portion 406 can be detachably coupled to permit removal, repair, and/or replacement of the front and/or rear portions 404 and 406, respectively, and/or to remove the rear portion 406 in response to high wind loads that may damage the turbine.

The front portion 404 of the airfoil 400 extends between the leading edge 408 and an interface surface 409 and the rear portion 406 extends from an interface surface 411 to the trailing edge 412. In some embodiments, the interface surface 411 of the rear portion 406 can interface with the interface surface 409 in the transition area 410 when the airfoil 400 is assembled such that the transition area 410 coincides with the interface surfaces 409 and 411. In some embodiments, the interface surfaces 409 and 411 can interface outside of the transition area 410 when the airfoil is assembled such that the transition area 410 does not coincide with the interface surfaces 409 and 411. In the example embodiment, surfaces 414 and 416 extend from the leading edge 408 of the airfoil 400 to the interface surface 409 of the front portion 404, and surfaces 415 and 417 extend from the interface surface 411 to the trailing edge 412.

A chord 422 defines a length of the airfoil 400 between the leading edge 408 and the trailing edge 412 of the airfoil 400, which can be determined based on a center or mean camber line 405 associated with the airfoil 400. The mean camber line 405 generally bisects the cross-section of the airfoil 400 midway between the outer surface of the airfoil 400 (e.g., the surfaces 414, 415) and the inner surface of the airfoil 400 (e.g., the surfaces 416, 417). The cross-sectional shape of the front portion 404 generally tapers away from the leading edge 408 and then tapers towards the mean camber line 405 to the transition area 410.

A cross-sectional thickness 424 of the front portion 404 of the airfoil 400 can correspond to distance from the outer surface 414 to the inner surface 416 of the front portion 404, measured perpendicular to and along the mean camber line 405. The cross-sectional thickness 424 of the front portion 404 is non-uniform and varies along the mean camber line 405 of the airfoil 400 from the leading edge 408 to the interface surface 409. A cross-sectional thickness 425 of the rear portion 404 of the airfoil 400 can correspond to distance from the outer surface 415 to the inner surface 417 of the rear portion 406, measured perpendicular to and along the mean camber line 405. The cross-sectional thickness 425 of the rear portion 406 is substantially uniform and constant along the mean camber line 405 of the airfoil 400 from the interface surface 411 to the trailing edge 412.

The cross-sectional thickness 424 of the front portion 404 varies along a length of the front portion such that an interior area 426 between the surfaces 414 and 416 changes along the mean camber line 405 from the leading edge 408 to the interface surface 409. The interior area 426 between the surfaces 414, 416 in the front portion 404 can be empty to form a hollow cavity, can include support structures, and/or can be filled with one or more materials, such as foam. In some embodiments, the rear portion 406 can have a slight taper towards the mean camber line at the trailing edge 412 to minimize turbulence of the fluid flows mixing beyond the trailing edge 412.

An attachment members can be disposed at the interface surface 409 of the front portion 404 to provide an interface to facilitate connection of the rear portion 406 to the front portion 404. For example, in the example embodiment, the interface surface 409 a recess 454 for receiving the interface surface 411 of the rear portion 406 and holes 455 extending through the outer surface 414 and inner surface 416 for receiving a bolt 456. The interface surface 411 of the rear portion 406 can be inserted into the recess 454 formed in the interface surface 409 of the front portion 404 and the trailing edge 412 can be positioned downstream of the interface surface 411.

The interface surface 411 of the rear portion 406 can have holes 472 corresponding to the holes 455 formed in the front portion 404 such that when the interface surface 411 of the rear portion 404 is inserted into the recess 454 of the interface surface 409 of the front portion 404, the holes 455 and 472 are aligned and configured to receive bolts 456. The bolts 456 can threadingly engage nuts 457 to secure the rear portion 404 to the front portion 406.

FIG. 8 illustrates another example embodiment of a multi-piece ringed shaped airfoil 500. The airfoil 500 can be suitable for use with fluid turbines, such as one or more fluid shrouded turbines described herein. The airfoil 500 can have a generally circumferential body 502 with a front portion 504 and a rear portion 506 formed as separate components that are joined by one or more attachment members, such as bolts, screws, rivets, clamps, clips, bracket, Velcro®, hinge, adhesives, welding, and the like. In some embodiments, the front portion 504 and the rear portion 506 can be detachably coupled to permit removal, repair, and/or replacement of the front and/or rear portions 504 and 506, respectively, and/or to remove the rear portion 506 in response to high wind loads that may damage the turbine.

As described above, the front portion 504 of the airfoil 500 extends between the leading edge 508 and an interface surface 509 and the rear portion 506 extends from an interface surface 511 to the trailing edge 512. In some embodiments, the interface surface 511 of the rear portion 506 can interface with the interface surface 509 in the transition area 510 when the airfoil 500 is assembled such that the transition area 510 coincides with the interface surfaces 509 and 511. In some embodiments, the interface surfaces 509 and 511 can interface outside of the transition area 510 when the airfoil is assembled such that the transition area 510 does not coincide with the interface surfaces 509 and 511. In the example embodiment, surfaces 514 and 516 extend from the leading edge 508 of the airfoil 500 to the interface surface 509 of the front portion 504, and surfaces 515 and 517 extend from the interface surface 511 to the trailing edge 512.

A chord 522 defines a length of the airfoil 500 between the leading edge 508 and the trailing edge 512 of the airfoil 500, which can be determined based on a center or mean camber line 505 associated with the airfoil 500. The mean camber line 505 generally bisects the cross-section of the airfoil 500 midway between the outer surface of the airfoil 500 (e.g., the surfaces 514, 515) and the inner surface of the airfoil 500 (e.g., the surfaces 516, 517). The cross-sectional shape of the front portion 504 generally tapers away from the leading edge 508 and then tapers towards the mean camber line 505 to the transition area 310.

A cross-sectional thickness 524 of the front portion 504 of the airfoil 500 can correspond to distance from the outer surface 514 to the inner surface 516 of the front portion 504, measured perpendicular to and along the mean camber line 505. The cross-sectional thickness 524 of the front portion is non-uniform and varies along the mean camber line 505 of the airfoil 500 from the leading edge 508 to the interface surface 509. A cross-sectional thickness 525 of the rear portion 504 of the airfoil 500 can correspond to distance from the outer surface 515 to the inner surface 517 of the rear portion 506, measured perpendicular to and along the mean camber line 305. The cross-sectional thickness 525 of the rear portion 506 is substantially uniform and constant along the mean camber line 505 of the airfoil 500 from the interface surface 511 to the trailing edge 512.

The cross-sectional thickness 524 of the front portion 504 varies along a length of the front portion 504 such that an interior area 526 between the surfaces 514 and 516 changes along the mean camber line 505 from the leading edge 508 to the interface surface 509. The interior area 526 between the surfaces 514, 516 in the front portion 504 can be empty to form a hollow cavity, can include support structures, and/or can be filled with a material, such as foam. In some embodiments, the rear portion 506 can have a slight taper towards the mean camber line 505 at the trailing edge 512 to minimize turbulence of the fluid flows mixing beyond the trailing edge 512.

An attachment member can be disposed at the interface surface 509 of the front portion 504 to provide an interface to facilitate connection of the rear portion 506 to the front portion 504. For example, in the example embodiment, the interface surface 509 can have a substantially planar rear surface 552 having one or more threaded holes 554 formed in the rear surface 552 for receiving bolts 556.

The rear portion 506 of the airfoil 500 can be formed of brackets 560 and a non-rigid or semi-rigid material 562, such as a fabric or polyethylene, which can be similar to a marine shrink wrap. The brackets 560 can have a generally L-shaped body having a first shorter extent 564 at a proximal end 566 of the brackets 560 and a second longer extent 568 disposed perpendicular to and extending from the first shorter extent 564 to a distal end of the brackets 570. The first shorter extent can include a hole 572 that corresponds to one of the holes 554 formed in the front portion 504 such that when the first shorter extent 564 of one of the brackets 560 is aligned with the rear surface 552 of the front portion 506 the holes 554 and 572 can be aligned and configured to receive one of the bolts 556. The bolts 556 can be used to secure the brackets 560 to the front portion 504 by threadingly engaging the holes 554 formed in the rear surface 552 of the front portion 504.

The non-rigid or semi-rigid material 562 can surround, encompass, or encircle the brackets 560 mounted to the rear surface 552 of the front portion 504. In some embodiments, the non-rigid or semi-rigid material 562 can be tacked, adhered, friction fit, or otherwise attached or fixed to the brackets 560 to secure the non-rigid or semi-rigid material 562 to the brackets 560. For example, for embodiments in which a polyethylene shrink wrap is used, the polyethylene shrink wrap can encircle the brackets 560 and heat can be applied to the polyethylene shrink wrap to shrink the polyethylene shrink wrap onto the brackets 560 forming a tight friction fit. Thus, the rear portion has a substantially uniform and constant thickness that is approximately equal to the thickness of the non-rigid or semi-rigid material 562.

While the example embodiment uses brackets to form a portion of the rear portion 506 of the airfoil 500, those skilled in the art will recognize that other implementation are possible. For example, in some embodiments, skeleton or framing members be integrally formed with and extending from an interface surface of the front portion 504 and the non-rigid or semi-rigid material can substantially encircle the skeleton or framing members. In some embodiments, the skeleton or framing members can be discretely spaced elongate members, similar to the brackets 560. In some embodiments, the skeleton or framing members can form a lattice or weave configuration to provide addition support and reinforcement to the non-rigid or semi-rigid material.

FIG. 9 illustrates another embodiment of a multi-piece ringed shaped airfoil 600 has a generally circumferential body (not shown) with a front portion 604 and a rear portion 606 formed as separate components that are joined by one or more attachment members, such as bolts, screws, rivets, clamps, clips, bracket, Velcro®, hinge, adhesives, welding, and the like. The airfoil 600 can be suitable for use with shrouded fluid turbines, such one or more of the shrouded fluid turbines described herein. In some embodiments, the front portion 604 and the rear portion 606 can be detachably coupled to permit removal, repair, and/or replacement of the front and/or rear portions 604 and 606, respectively, and/or to remove the rear portion 606 in response to high wind loads that may damage the turbine.

The front portion 604 of the airfoil 600 extends between the leading edge 608 and an interface surface 609 and the rear portion 606 extends from an interface surface 611 to the trailing edge 612. In some embodiments, the interface surface 611 of the rear portion 606 can interface with the interface surface 609 in the transition area 610 when the airfoil 600 is assembled such that the transition area 610 coincides with the interface surfaces 609 and 611. In some embodiments, the interface surfaces 609 and 611 can interface outside of the transition area 610 when the airfoil is assembled such that the transition area 610 does not coincide with the interface surfaces 609 and 611. In the example embodiment, surfaces 614 and 416 extend from the leading edge 608 of the airfoil 600 to the interface surface 609 of the front portion 604, and surfaces 615 and 617 extend from the interface surface 611 to the trailing edge 612.

A chord 622 defines a length of the airfoil 600 between the leading edge 608 and the trailing edge 612 of the airfoil 600, which can be determined based on a center or mean camber line 605 associated with the airfoil 600. The mean camber line 605 generally bisects the cross-section of the airfoil 600 midway between the outer surface of the airfoil 600 (e.g., the surfaces 614, 615) and the inner surface of the airfoil 600 (e.g., the surfaces 616, 617). The cross-sectional shape of the front portion 604 generally tapers away from the leading edge 608 and then tapers towards the mean camber line 605 to the transition area 610.

A cross-sectional thickness 624 of the front portion 604 of the airfoil 600 can correspond to distance from the outer surface 614 to the inner surface 616 of the front portion 604, measured perpendicular to and along the mean camber line 605. The cross-sectional thickness 624 of the front portion 604 is non-uniform and varies along the mean camber line 605 of the airfoil 600 from the leading edge 608 to the interface surface 609. A cross-sectional thickness 625 of the rear portion 604 of the airfoil 600 can correspond to distance from the outer surface 615 to the inner surface 617 of the rear portion 606, measured perpendicular to and along the mean camber line 605. The cross-sectional thickness 625 of the rear portion 606 is substantially uniform and constant along the mean camber line 605 of the airfoil 600 from the interface surface 611 to the trailing edge 612.

The cross-sectional thickness 624 of the front portion 604 varies along a length of the front portion 604 such that an interior area 626 between the surfaces 614 and 616 changes along the mean camber line 605 from the leading edge 608 to the interface surface 609. The interior area 626 between the surfaces 614, 616 in the front portion 604 can be empty to form a hollow cavity, can include support structures, and/or can be filled with one or more materials, such as foam. In some embodiments, the rear portion 606 can have a slight taper towards the mean camber line 605 at the trailing edge 612 to minimize turbulence of the fluid flows mixing beyond the trailing edge 612.

A pivot joint 601, which can form a flexible, pivotal, or hinge-like attachment member, can be disposed between the interface surface 609 of the front portion 604 and the interface surface 611 of the rear portion 606 to provide an interface for connection of the rear portion 606 to the front portion 604 and to permit the rear portion 606 to be moveable relative to the front portion 604, or vice versa. For example, the flexible, pivotal, hinge like joint 601 can permit active (controlled) or passive relative motion between the front portion 304 and the rear portion 306. As shown in phantom in FIG. 8, the rear portion 606 can move, for example, between a position 690 and a position 692.

FIGS. 10-12 depict another example shrouded fluid turbine 700 that includes a turbine shroud 704, the ejector shroud 106 downstream of the turbine shroud 704, the nacelle body 116, and the rotor 110. As illustrated by FIGS. 10 and 11, the rotor 110, turbine shroud 704, and ejector shroud 106 may be coaxial with each other, (i.e., all share the common central axis 705). The turbine shroud 704 can be formed in a similar manner as the turbine shroud 104. However, the mixing lobes 726, 728 differ from the mixing lobes 126, 128, and the airfoil shape defined by the turbine shroud 704 can differ from the airfoil shape of the turbine shroud 104. The mixing lobes 726, 728 of the turbine shroud 704 can include two sets of separate and distinctly formed mixing lobes such that there are no radially extending side walls connecting low energy mixing lobes 728 and high energy lobes 726. The arrangement of the mixing lobes 726, 728 can facilitate circumferential mixing of fluid flow as well as both lateral and axial mixing of fluid flow.

With respect to FIG. 12, the turbine shroud 704 has the cross-sectional shape of an airfoil similar to the cross-sectional shape of the airfoil defined by the ejector shroud 106 in that the airfoil defined by the turbine shroud 704 includes a front portion 780 and a rear portion 782. The airfoil defined by the turbine shroud 704 can have a generally cylindrical or ring-like configuration having a circumferential body extending radially about the rotor 110. The airfoil can have a leading edge 722 and a trailing edge 724. A front portion 780 of the airfoil can extend from the leading edge 722 to a transition area 723. The leading edge 722 of the front portion 780 can be generally rounded, bull-nosed, or otherwise shaped to form a surface for dividing a fluid flow into at least two separate flows or streams. The front portion 780 can have an outer surface 734 and an inner surface 736 extending between the leading edge 722 and the transition area 723 to form an interior area 760 having a non-uniform cross-sectional thickness 738 along a mean camber line 733. The mean camber line 733 generally bisects the cross-section of the airfoil midway between the outer surface of the airfoil and the inner surface of the airfoil. The cross-sectional thickness 738 of the front portion 780 is measured between the surfaces 734 and 736 and along the mean camber line 733.

The cross-sectional shape of the front portion 780 generally tapers away from the leading edge 722 and then tapers towards the mean camber line 733 to the transition area 723. The cross-sectional thickness 738 of the front portion 780 generally varies along a length of the front portion 780 from the leading edge 722 to the transition area 723. The rear portion 782 of the airfoil defined by the turbine shroud 104 can extend from the transition area 723 to the trailing edge 724. The rear portion 782 can have a substantially uniform and constant cross-sectional thickness 739 along the mean camber line from the transition area 723 to the trailing edge 724.

FIGS. 13-16 depict another example shrouded fluid turbine 800 that includes a turbine shroud 804, the ejector shroud 106 downstream of the turbine shroud 804, the nacelle body 116, and the rotor 110. As illustrated by FIGS. 13 and 14, the rotor 110, turbine shroud 804, and ejector shroud 106 may be coaxial with each other, (i.e., all share the common central axis 805). The turbine shroud 804 can be formed in a similar manner as the turbine shrouds 104 (FIGS. 1-3) and 704 (FIGS. 10-12). The turbine shroud 804 has a cross-sectional shape of an airfoil similar to the cross-sectional shape of the airfoil defined by the turbine shroud 704. The mixing lobes 826, 828 differ from the mixing lobes 726, 728. The mixing lobes 826, 828 are similar to the mixing lobes 126, 128 of FIGS. 1-3.

Referring to FIG. 16, which is a rear view of the fluid turbine of FIG. 11, the structure of the mixing lobes 826, 828 can be more easily explained. The turbine shroud 104 has a throat diameter D_(F). This throat diameter D_(F) is measured as the smallest diameter of the turbine shroud, and is generally located near the rotor 110. The trailing edge 824 of the turbine shroud 804 has an inner diameter D_(RI) and an outer diameter D_(RO). The inner diameter D_(RI) is measured as the diameter of a circle formed by the trailing edges of the high energy mixing lobes 826. Similarly, the outer diameter D_(RO) is measured as the diameter of a circle formed by the low energy mixing lobes 828. It should be recognized that generally D_(F)>D_(RI), D_(RO)>D_(RI), and D_(RO)>D_(F). The ejector shroud 104 can also have a throat diameter D_(I). This throat diameter D_(I) can be measured as the smallest diameter of the ejector shroud, and is generally located near the leading edge 150 of the ejector shroud. The ejector shroud 104 also has an outer diameter D_(E) at the trailing edge 152. This outer diameter is measured as the diameter or width of a generally circular configuration formed by the trailing edge 152 of the ejector shroud 104.

The trailing edge 824 of the turbine shroud 804 has a generally circular crenellated shape when view from the rear. The trailing edge 824 can be described as including several inner circumferentially spaced arcuate portions 894 which each can have the same radius of curvature. The inner arcuate portions 894 can be evenly spaced apart from each other. Between portions 894 are several outer arcuate portions 896, which each can have the same radius of curvature. The radius of curvature for the inner arcuate portions 894 can be different from the radius of curvature for the outer arcuate portions 896, but the inner arcuate portions and outer arcuate portions can have the same center (i.e. along the central axis). The inner arcuate portions 894 and the outer arcuate portions 896 are then connected to each other by radially extending portions 895. The term “crenellated” as used herein does not require the inner arcuate portions, outer arcuate portions, and radially extending portions to be straight lines, but instead refers to the general circumferential undulating or in-and-out shape of the trailing edge of the turbine shroud 804. This crenellated structure forms two sets of mixing lobes, high energy mixing lobes 826 and low energy mixing lobes 828. As shown in FIGS. 13, 14, and 16, the mixing lobes 826, 828 have full sidewalls so that fluid flowing through the mixing lobes can flow axially.

FIGS. 17-19 depict another example shrouded fluid turbine 900 that includes a turbine shroud 904, the ejector shroud 106 downstream of the turbine shroud 004, the nacelle body 116, and the rotor 110. As illustrated by FIGS. 17 and 18, the rotor 110, turbine shroud 904, and ejector shroud 106 may be coaxial with each other, (i.e., all share the common central axis 905). In the example embodiment, front and rear portions 980 and 982, respectively can be formed as separate components that are joined form the turbine shroud 904, and front and rear portions 146 and 148, respectively, can be formed as separate components that are joined to form the ejector shroud 104. The rear portion 148 of the ejector shroud 104 can be composed a fabric or polymeric film, a sheet of metal, and/or sheet of composite. Likewise, mixing lobes 926, 928 (i.e., a rear portion 982) can also be composed of fabric, polymeric film, one or more sheets of metal, and/or one or more sheets of a composite. For embodiments in which a fabric is used, the fabric can be impregnated with a polymer resin such as PVC or a polymer film form, such as modified polytetrafluoroethylene (PTFE).

An outer terminus ring 994 defines a trailing edge of the low energy lobes 928, where the low energy lobes 928 are attached to the outer terminus ring 994. An inner terminus ring 990 defines a trailing edge of the high energy lobes 926, where the trailing edge of the high energy mixing lobes 926 are attached to the inner terminus ring 990. In the example, as shown in FIGS. 17-19, front portions 980 and 946 of the turbine shroud 904 and the ejector shroud 104, respectively, can have a sandwich composite construction with a foam core and a composite surface on either side, such as an epoxy impregnated e-glass matte that yields a high beam stiffness construction with overall low density.

Although a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this disclosure. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. 

We claim:
 1. An airfoil for an energy extracting shrouded fluid turbine comprising: a front portion having a generally circumferential body extending circumferentially about a central axis and defining a leading edge of the airfoil, the front portion having a non-uniform cross-sectional thickness extending along a mean camber line to a transition area; and a rear portion having a generally circumferential body extending circumferentially about the central axis and defining a trailing edge of the airfoil, the rear portion having a substantially uniform cross-sectional thickness extending along the mean camber line from the transition area to the trailing edge.
 2. The airfoil of claim 1, wherein the front portion and the rear portion are integrally formed.
 3. The airfoil of claim 1, wherein the front portion and the rear portion are separately formed and joined to form the airfoil.
 4. The airfoil of claim 4, wherein the rear portion is detachably coupled to the front portion.
 5. The airfoil of claim 4, wherein the front portion includes a first surface and a second surface extending between the leading edge and the transition area, the first and second surface being spaced apart from each other to form an interior area, and the rear portion includes a third surface and a fourth surface extending between the transition area and the trialing edge, the third and fourth surface substantially abutting each other over a length of the rear portion.
 6. The airfoil of claim 5, wherein the interior area of the front portion is at least one of hollow and filled with foam.
 7. The airfoil of claim 5, wherein the cross-sectional thickness of the rear portion is approximately a thickness of the third and fourth surfaces.
 8. The airfoil of claim 5, wherein the first and third surfaces are continuously formed and the second and fourth surfaces are continuously formed.
 9. The airfoil of claim 1, wherein the front portion and the rear portion are formed of different materials.
 10. The airfoil of claim 1, wherein the rear portion comprises a non-rigid material.
 11. The airfoil of claim 1, wherein a cross-sectional shape of the front portion generally tapers away from the leading edge and then tapers towards the mean camber line to the transition area along the mean camber line.
 12. An energy extracting shrouded fluid turbine comprising: an energy extracting assembly including a rotor; and an airfoil having a generally circumferential body disposed radially about the rotor, the airfoil including a front portion extending between a leading edge of the air foil and a transition area of the airfoil and a rear portion extending between the transition area and a trailing edge of the airfoil, the front portion having a non-uniform cross-sectional thickness extending along a mean camber line of the airfoil and the rear portion having a substantially uniform cross-sectional thickness extending along the mean camber line.
 13. The turbine of claim 12, wherein the airfoil is an ejector shroud positioned downstream of the rotor.
 14. The turbine of claim 13, further comprising a turbine shroud having an inlet end positioned upstream of the rotor to form a fluid flow through the shroud and passed the rotor, and an exhaust end positioned downstream of the rotor and disposed within the ejector shroud, the exhaust end including a mixing lobe to manipulate the fluid flow.
 15. The turbine of claim 12, wherein the airfoil is a turbine shroud and the rear portion comprises mixing lobes to manipulate fluid flow through the turbine.
 16. The turbine of claim 12, wherein the front portion and the rear portion are formed of different materials.
 17. The turbine of claim 12, wherein a cross-sectional shape of the front portion varies and generally tapers away from the leading edge and then towards the mean camber line to the transition area along the mean camber line.
 18. The turbine of claim 15, wherein the rear portion is formed of a fabric.
 19. A airfoil for an energy extracting shrouded fluid turbine comprising a generally circumferential body extending circumferentially about a central axis and defining a leading edge and a trailing edge of the airfoil; a mean camber line extending between the leading edge and the trailing edge, the mean camber line bisecting a cross-section of the airfoil; a varied cross-sectional thickness along the mean camber line between the leading edge and a transition area, the varied cross-sectional thickness varying and generally decreases towards the transition area; and a uniform cross-sectional thickness along the mean camber line between the transition area and the trailing edge, the uniform cross-sectional thickness being substantially constant.
 20. The airfoil of claim 19, wherein a front portion is defined between the leading edge and the transition area and a rear portion is defined between the transition area and the trailing edge, the front and rear portions being separate and distinct components that are attached to form the airfoil and are formed of different materials. 